The present invention relates to an apparatus for holding an electrode, in particular a cylindrical, tubular electrode for an electrical erosion machine, which operates on the principle of spark-erosion machining.
In spark-erosion machining, the electrode is moved on a chosen track and the desired material erosion on a workpiece takes place by means of a spark discharge between this electrode and the workpiece. In the machining process, the electrode is continuously subject to wear, which leads to the electrode becoming shorter.
For spark-erosion machining, as described, for example, in U.S. Pat. No. 5,561,336, it is advantageous for the tool to be a tubular electrode which is located in a holder which, in turn, is held by a spindle which carries out a rotational movement. The principle of spark-erosion machining is preferably suitable for electrodes of small diameter. In a corresponding manner, the spindle and the electrode holder should have small dimensions in order to avoid unbalance problems, as well as problems with collisions between the electrode holder or spindle and the workpiece.
Conventionally available tool holders for use in machine tools are in general designed for transmitting relatively large torques and for withstanding the reaction. In this case, cone-cone couplings are used between the spindle and tool holder. The corresponding axial closing force which draws the tip of a cone of a tool holder into the corresponding spindle is thus very large (of the order of magnitude of 500 kg or more). It is also used to compensate for any play that there may be between the tool cone and the spindle cone. The force is sufficient to lengthen the cone of the tool holder and, as a consequence of this, to deform it. Since the spindle should be correspondingly small for electrodes with a small diameter, only small closing forces can be applied there. In consequence, any existing play cannot be compensated for by lengthening the cone of the tool holder.
Copper is preferably used as the electrode material, being soft and flexible; however, this property is disadvantageous for firmly clamping the electrode into a conventional holder, since such holders are designed for convention cutting tools, which are stiff and rigid.
Owing to wear, the electrodes frequently have to be replaced, preferably automatically. In the process, the worn electrode can be replaced either by replacement with the holder from the spindle or only by replacement of the electrode in the holder, with the new electrode being repositioned in the same holder. In order to continue the machining process without any problems after it has been necessary to replace an electrode, the free electrode end must be repositioned within a tolerance of plus or minus 0.01 mm. The precision with which a conventional cone-cone coupling can be repositioned is inadequate for this purpose and, in particular, an improvement in precision for a series of cones would be very costly.
Furthermore, very good sealing for the dielectric working liquid must be ensured at the connection points from the electrode to the holder and from the holder to the spindle. This liquid is introduced at a controlled pressure through the electrode in order to evacuate the erosion particles and is required in the formation of the desired spark discharge. In the case of a conventional tool holder, it is unnecessary to introduce working liquid through the tool, and, therefore, there are no sealing problems.
Accordingly, it is the principle object of the present invention to provide an electrode holder for erosion machines working on the principle of spark-erosion machining, which allows for an electrode to be fixed and for rotational movement to be carried out with good reproduction accuracy, wherein the electrode preferably comprises a small diameter, cylindrical copper tube. The holder is intended to ensure simple handling, and, in particular, the electrodes should not be damaged while being fitted to or removed from the holder.